1. Field of the Invention
This invention relates to a probe tracing method and means for a coordinate measuring machine, and more particularly to a touch probe which electrically outputs touch signals by means of contacts with an object to be measured.
2. Description of the Prior Art
The coordinate measuring machine is well-known as the machine which accurately measures the external dimension of an object with complicated shape, and is well-known in various industrial fields as a two or three dimension measuring machine.
In FIG. 1 shown therein is a well-known three dimension coordinate measuring machine, and an object to be measured is fixingly held on a bed 10. The size of an object 12 can be measured with high accuracy by reading such coordinate portions of a touch probe 14 that the probe 14 encounters to the external surface of the object to be measured at predetermined measuring points. In order to move and position the touch probe in optional X, Y and Z directions, the measuring machine is provided with an X axial motor 16, a Y axial motor 18 and a Z axial motor 20. A probe tracing device is provided with the motors 16, 18 and 20, and applies trace control signals having each of the speed signals and the directional signal from a trace control means 22 to the motors so that the probe 14 can be moved to predetermined directions to encounter the object 12 at the predetermined measuring points. The touch probe outputs touch signals 100 when it makes contact with the object 12 to be measured. The touch signals 100 are supplied to a signal processing circuit, which is not illustrated, by way of a terminal 24 to read X, Y and Z coordinates at each of the measured portions. Thus, the external size of the object 12 can be measured and recorded. The above mentioned trace control means 22 can be formed with a manual control means wherein an operator inclines a remote control lever toward requested directions to output the voltage change at this time corresponding to trace control value, JOYSTICK for example, or can be formed with an automatic control means outputting predetermined measuring program.
Incidentally, the speed signal consists of pulse signals, and can rotate the motors a certain amount per one pulse. Accordingly, the pulse density during a certain period is proportional to the rotating speed of the motors.
The touch probe 14 used for the coordinate measuring machine described in the above has a displacement mechanism in its contact assembly, and this mechanism prevents the touch probe 14 from destruction in case of movement of the touch probe 14 toward the object to be measured by means of the force of inertia after the touch probe 14 encounters the object 12.
After the touch probe 14 makes contact with the object 12 and completes the measurement, it is controlled to move toward a next measuring point. In a return stroke from the object 12 in the prior art device there is such a problem that the touch probe 14 collides with the object 12. Accordingly, such collision of the touch probe 14 with the object 12 results in deformation, destruction or loss of the expensive touch probe as well as inconvenience since the measuring action must be repeated all over again from the beginning.
In the prior art device illustrated in FIG. 2, particularly, there are found such drawbacks that the touch probe 14 cannot be moved back from the measuring portions in blind locations on the bore portion of the object 12.
The collision of the touch probe 14 with the object 12 in the return stroke mentioned above can be avoided by setting the return stroke to be the same as the proceeding contact stroke. It is, however, extremely difficult in a manual control means wherein the control lever is inclined toward the completely opposite position from the previous inclined position in the contact state of the touch probe 14 to the object 12, and operational error of the control lever frequently causes the collision mentioned in the above. Furthermore, since in the automatic control means in accordance with a predetermined program the tracing control in the actual contact is performed by feedback control with the accompanyment of tracing stroke path drift which changes everytime, it is difficult to completely program such proceeding contact smoke beforehand and it is impossible to establish the return stroke in the same way as the proceeding contact stroke.